In recent years, spatial light modulators have been widely applied to aberration compensation in ultrafast laser processing, multi-focus parallel machining, two-dimensional surface machining, three-dimensional bulk machining, pulse temporal and spatial shaping, and structured light machining. Here the working principle of spatial light modulators is introduced. Hologram generation algorithms of spatial light modulators are explained. The applications of spatial light modulators to the ultrafast laser processing field are elaborated.